1. Field of the Invention
The present invention relates to control devices for hydraulic winches for controlling winding-up/winding-down operations of winch drums by hydraulic motors having variable capacity functioning as power sources.
2. Description of the Related Art
A hydraulic motor having variable capacity is often used as a driving source of a hydraulic winch for varying speed and power of winding-up/winding-down in response to a load. The structure of an exemplary device is shown in FIG. 6.
A negative brake 12 for maintaining a hydraulic motor 1 in a halt state is provided on the hydraulic motor 1. This negative brake 12 is activated when a brake valve 14 shifts from a brake-releasing position x to a brake-activating position y to release the hydraulic pressure in a pressure chamber 12a into a tank T.
A switching valve 16 is controlled by a signal from a controller 11. At the time of an automatic shutoff, the switching valve 16 shifts from a readout position y for reading out a remote-control pressure to a shutoff position x for shutting off the remote-control pressure.
A regulator 18 is fundamentally controlled on the basis of two signals including a load pressure applied to the hydraulic motor 1 and the amount of the operation of a remote-control valve 6. The “load pressure” means the absolute value of a difference in pressure between the inlet and the outlet of the motor. The differential pressure herein is determined by subtracting the pressure at the winding-down side pipeline 3 from that at the winding-up side pipeline 2.
Specifically, the regulator 18 transmits the load pressure via load pressure lines 19, and the motor capacity is increased with the increase of the load pressure by the operation of a sequence valve (not shown) or a constant horsepower (CHP) valve (not shown). Accordingly, the increase of the load pressure is regulated (constant-horsepower control).
Secondly, remote-control pressure lines 7u and 7d are connected to the regulator 18 via a shuttle valve 17 and a readout line 20 for reading out the remote-control pressure. With this arrangement, the motor capacity is decreased as the amount of the operation of the remote-control valve 6 is increased, and thus, the motor speed is increased (motor-speed control).
In addition, when the amount of the operation of the remote-control valve 6 is zero, i.e. in a neutral state, the motor capacity is set to the maximum.
However, the above-described structure has the following problems:
(i) Slow Control Response
For example, during winding-up of a large load, combined control of lowering a boom and winding-up with a winch can cause the load to swing. In this case, since the load fluctuates around a border of an overload level, chattering occurs to repeat the automatic shutoff and releasing the automatic shutoff.
If the remote-control valve 6 is returned to the neutral position at this time, the hydraulic motor 1 is set to a large capacity. On the contrary, if the winding-up operation is continued, the negative brake 12 is activated at the automatic shutoff, and the load pressure is set to zero. As a result, the motor capacity is set at a small value.
Accordingly, when the negative brake 12 is released, a certain time is required for the motor capacity to return to a required value depending on the load pressure at that time.
Therefore, a high load pressure is temporally applied to the small-capacity motor at the time of returning from the automatic shutoff to cause a slow control response.
(ii) Low Motor-Capacity Ratio
FIG. 7 illustrates the relationship between a single line pull of a winch (load pressure) and a single line speed (motor capacity). The curved portion in FIG. 7 shows a control range in a constant horsepower.
For example, a motor-capacity range of the hydraulic motor 1 is defined between a point B (smaller capacity) and a point C (larger capacity) in the medium capacity range (the range between broken lines). When the motor is automatically halted at the larger capacity (point C) during suspending of a load, the negative brake 12 is activated to set the load pressure to zero. Consequently, the motor capacity is reduced to the smaller value (point B) due to the constant horsepower control.
When the automatic shutoff is released while the remote-control valve 6 is operated, the motor is instantaneously subjected to the load at the point C with the capacity at the point B. The load pressure at this time is expressed by RC/B×P, where RC/B is the motor-capacity ratio determined by dividing the motor capacity at the point C by the motor capacity at the point B, and P is a predetermined pressure for constant horsepower control.
For example, when P is set at half of a predetermined pressure of an overload-relief valve 9 (overload pressure) and RC/B is 2 or less, the load pressure is less than the overload pressure. Therefore, the motor capacity increases from the point B to the point C without an activation of an overload-relief operation.
In contrast, when the motor capacity ranges from a point A (minimum capacity) to the point C, for example, the motor-capacity ratio is increased, and thus the load pressure at the time of returning from the automatic shutoff increases to RC/A×P, where RC/A is the motor-capacity ratio determined by dividing the motor capacity at the point C by the motor capacity at the point A. In this case, the load pressure is higher than the overload pressure, and the overload-relief operation is activated. Accordingly, the control response to winding-up is very slow.
This is one of the reasons why the motor-capacity ratio of the hydraulic motor 1 cannot be increased. As a result, the speed control range at the same amount of supplied oil cannot be expanded.